What is the Treskilling Yellow that was sold at auction for over $2 million in 1996?

The Treskilling Yellow is a Swedish postage stamp that, as of 2014, recorded the highest value at auction of any stamp. In 1855, Sweden accidentally printed its 3-skilling stamp with yellow ink rather than the correct green color. Only one copy of the stamp is known to exist.

The price of the stamp has successively increased over time. In 1984, it was sold for $1.07 million, and in 1996, it was sold for approximately $2.3 million. It was sold again in 2010 for an undisclosed price, although the auctioneer confirmed that, once again, the stamp was sold at a higher price than any other rare stamp.